Various powder coatings have been known that consist basically of a uniform mixture of an epoxy resin, a carboxylic acid anhydride in powder form, and a filler. These powder coatings are applied by the fluidized-bed coating process, the electrostatic fluidized-bed coating process or some other suitable methods to form insulation films on electrical and electronic devices.
Powder coatings are either melt-mixed or dry-blended. In the first process, dry ingredients including an epoxy resin base and additives such as a curing agent and a filler are mixed; the premix is then melt-compounded, cooled to solidify, finely ground, and classified. In the second process, the premix of dry ingredients is pressure-compounded, finely ground and classified. If a high-melting point polycarboxylic acid anhydride used as the curing agent poorly mixes and disperses in the resin, a product having desired coating performance cannot be attained. If a low-melting point monocarboxylic acid anhydride is additionally used in an attempt to improve the dispersive miscibility of the polycarboxylic acid anhydride with the resin, the anti-blocking property and storage stability of the final product will be impaired. If the polycarboxylic acid anhydride is mixed and dispersed in the resin at elevated temperatures, the storage stability of the final product is impaired.